Why do onions make me cry? © Alamy

Why do onions make me cry?

Asked by: Christian (aged 10), Hertfordshire

The onion’s cells contain an enzyme called alliinase and chemical compounds called sulfoxides. Normally the alliinase and sulfoxides are kept apart, but when the onion is chopped, they are released and react to form sulfenic acid.

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This acid is then acted on by a second type of enzyme called a synthase, which produces a gas called ‘syn-propanethial-S-oxide’. This diffuses through the air and irritates your eyes, prompting your tear glands to produce tears to flush it out.

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